Method and apparatus for performing target-image-based optical proximity correction

ABSTRACT

A system that performs target-image-based optical proximity correction on masks that are used to generate an integrated circuit is presented. The system operates by first receiving a plurality of masks that are used to expose features on the integrated circuit. Next, the system computes a target image for a target feature defined by the plurality of masks, wherein mask features from different masks define the target image. The system dissects the feature into a plurality of segments, wherein dissecting the mask feature involves using dissection parameters associated with geometric characteristics of the target image, instead of using dissection parameters associated with geometric characteristics of the mask feature. The system then performs an optical proximity correction (OPC) operation on the plurality of masks, wherein the OPC operation uses parameters associated with geometric characteristics of the target image to perform optical proximity correction on the mask features that define the target image.

BACKGROUND

1. Field of the Invention

The invention relates to the process of fabricating integrated circuits.More specifically, the invention relates to a method and an apparatusfor performing target-image-based proximity correction for advancedphotomasks during fabrication of an integrated circuit.

2. Related Art

Recent advances in integrated circuit technology have largely beenaccomplished by decreasing the feature size of circuit elements on asemiconductor chip. As the feature size of these circuit elementscontinues to decrease, circuit designers are forced to deal withproblems that arise as a consequence of the optical lithography processthat is typically used to manufacture integrated circuits. This opticallithography process begins with the formation of a photoresist layer onthe surface of a semiconductor wafer. A mask composed of opaque regions,which are generally formed of chrome, and light-transmissive clearregions, which are generally formed of quartz, is then positioned overthis photoresist layer. (Note that the term “mask” as used in thisspecification is meant to include the term “reticle.”) Light is thenshone on the mask from a visible light source, an ultraviolet lightsource, or more generally some type of electromagnetic radiation sourcetogether with suitably adapted masks and lithography equipment.

This image is reduced and focused through an optical system containing anumber of lenses, filters, and mirrors. The light passes through theclear regions of the mask and exposes the underlying photoresist layer.At the same time, opaque regions of the mask block the light leavingunderlying portions of the photoresist layer unexposed.

The exposed photoresist layer is then developed, through chemicalremoval of either the exposed or non-exposed regions of the photoresistlayer. The end result is a semiconductor wafer with a photoresist layerhaving a desired pattern. This pattern can then be used for etchingunderlying regions of the wafer.

As integration densities continue to increase, it is becoming necessaryto use phase-shifters to define more and more features within a layout.In addition to the mask containing these phase-shifters, a separate trimmask is used to selectively expose portions of the photoresist tocorrect (trim) the exposed areas of the photoresist to more accuratelyform the desired features of the integrated circuit. This is commonlyknown as a double exposure alternating aperture phase shift mask(AAPSM).

One problem that occurs during the optical lithography process is cornerrounding. Comer rounding is caused by optical effects such as lightbeing diffracted around the corner, and by uneven etching of thephotoresist at the corner. Outside corners tend to cause the exposedimage to be rounded inward, whereas inside corners tend to cause theexposed image to be rounded outward. An optical proximity correction(OPC) process adds corrections or serifs to the corners in the masks sothat the exposed image comes closer to agreeing with the target image.As a convenience, the term OPC is used in the most general sense toencompass any and all types of proximity corrections performed as partof the design and lithography process. In any given design flow, theuser may have an option to control which of the different types ofproximity correction the OPC software will apply.

For conventional simple photomasks such as binary masks, the mask imagecorresponds well to the intended target wafer image. For example, anintended corner in the wafer appears to be a corner as well on the mask.Hence, the corresponding OPC strategies have mostly been focused on themask features. However, in advanced photomasks such as double exposureAAPSM, the mask images do not directly correspond to the desired waferimage. An OPC process that targets the mask image or feature may not beeffective as for a simple photomask as seen in the example below.

FIG. 1A illustrates a phase shift mask 102 for creating a feature on anintegrated circuit. Phase shift mask 102 includes 0-degree phase shifter104 and 180-degree phase shifter 106. During operation, electromagneticradiation is passed through the 0-degree phase shifter 104 and180-degree phase shifter 106. Optical interference caused by thedifference in phase of the shifters causes the underlying photoresistbetween the two shifters to be left unexposed to help form a printedfeature on an integrated circuit.

FIG. 1B illustrates a binary or trim mask used in conjunction with thephase shift mask for creating a feature on an integrated circuit. Binarymask 108 includes trim feature 110. Trim feature 110 can be formed froman opaque material such as chrome. During a second exposure of thephotoresist using binary mask 108, the printed feature on the integratedcircuit is further defined. The remaining areas of the mask 108 mayinclude phase shifting features and/or attenuated material asappropriate.

FIG. 1C illustrates a desired feature 120 (lighter stipple andrectangular solid outline) and a printed feature 118 (darker stipple andcurved solid outline) created by using the phase shift mask 102 and thebinary mask 108 to expose a wafer. Desired feature 120 represents thedesired feature to be printed on the integrated circuit, while printedfeature 118 represents what is actually printed on the integratedcircuit. The outlines of 0-degree phases shifter 104, 180-degree phaseshifter 106, and trim feature 110 are shown for reference. Printedfeature 118 includes several areas that differ from desired feature 120.In line end area pullback and rounding of the end of printed feature 118caused by diffraction effects and resist effects is visible. Area 112shows bulges in printed feature 118 caused by corner effects of phaseshifters 104 and 106, and trim feature 110. Area 116 shows portions ofprinted feature 118 that are too narrow, possibly caused by edgediffraction effects. An optical proximity correction (OPC) process istherefore used in an attempt to make printed feature 118 more closelymatch desired feature 120 as described below in conjunction with FIGS.2A–D.

FIG. 2A illustrates edges 202 on phase shifters 104 and 106 and edges204 on trim feature 110 used to create a printed feature on anintegrated circuit (both shown with thicker solid lines). Note thatneither phase shifters 104 and 106 nor trim feature 110 individuallydescribes the desired printed feature. The desired printed feature isevident only from the combination of exposing the photoresist throughphase shifters 104 and 106, and trim feature 110.

FIG. 2B illustrates dissection points and evaluation points on phaseshifter edges 202 and trim feature edges 204. During a traditional, maskimage-based OPC process, edges 202 and 204 are dissected as illustratedat 206 and evaluation points are selected as illustrated at 208. Notethat only some dissection points and evaluation points are numbered forsimplicity. Short horizontal lines indicate the dissection points, whilean “X” indicates evaluation points. During the OPC process eachsegmented edge based on the dissection points will be biased in or outaccording to computations made at the evaluation point to help the finalimage more closely approximate the target layout.

FIG. 2C illustrates the results of optical proximity correction on phaseshifters 104 and 106, and trim feature 110. The OPC process followsshifter corner-to-corner minimum spacing rules at location 210. Shiftercorner-to-corner minimum spacing rules are mask making requirements, andare also used sometimes to prevent certain potential printabilityproblems. According to the mask images, areas 212 are corners and henceare corrected using tolerances and thresholds specified for corners.Hammerhead 216 is used to alleviate line end shortening problems.Similar to shifter corner-to-corner minimum spacing rules, trim cornerminimum width rules apply at area 214.

FIG. 2D illustrates a printed feature 118 created from the correctedphase shifters 104 and 106, and from trim feature 110. Outlines forphase shifters 104 and 106 and trim feature 110 are shown for reference.Note that while the line ends and line edges are closer to desiredfeature 120, there is still a bulge at 218 due to insufficientcompensation for the corner rounding effects of both masks.

Hence, what is needed is a method and an apparatus for performingoptical proximity correction while minimizing the problems describedabove.

SUMMARY

One embodiment of the invention provides a system that performstarget-image-based optical proximity correction on masks that are usedto generate an integrated circuit. The system operates by firstreceiving a plurality of masks that are used to expose features on theintegrated circuit. Next, the system computes a target image for atarget feature defined by the plurality of masks, wherein mask featuresfrom different masks define the target image. The system then dissectsthe mask feature into a plurality of segments, wherein dissecting themask feature involves using dissection parameters associated withgeometric characteristics of the target image, instead of usingdissection parameters associated with geometric characteristics of themask feature. The system then performs an optical proximity correction(OPC) operation on the plurality of masks, wherein the OPC operationuses parameters associated with geometric characteristics of the targetimage to perform optical proximity correction on the mask features thatdefine the target image.

In a variation on this embodiment, different OPC parameters areassociated with line edges, space edges, outer corners, inner corners,line ends, and slot ends of the target image.

In a variation on this embodiment, performing optical proximitycorrection on a mask feature involves placing evaluation points on theplurality of segments, and using the dissection and the associatedevaluation points in performing an OPC operation on the feature.

In a further variation, placing evaluation points on the mask featureinvolves using evaluation point placement rules associated withgeometric characteristics of the target image, instead of usingevaluation point placement rules associated with geometriccharacteristics of the mask feature.

In a further variation, performing an OPC operation on the mask featureinvolves using a resist threshold associated with geometriccharacteristics of the target feature instead of geometriccharacteristics of the mask feature.

In a further variation, performing an OPC operation on the mask featureinvolves using tolerances associated with geometric characteristics ofthe target feature instead of tolerances associated with geometriccharacteristics of the mask feature.

In a variation of this embodiment, the set of different masks includesat least one phase-shift mask and at least one trim mask.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A illustrates a phase shift mask for creating a feature on anintegrated circuit.

FIG. 1B illustrates a binary or trim mask used in conjunction with thephase shift mask of FIG. 1A for creating a feature on an integratedcircuit.

FIG. 1C illustrates a desired feature and a printed feature created byusing the phase shift mask of FIG. 1A and the binary mask of FIG. 1B.

FIG. 2A illustrates edges on phase shifters and trim features used tocreate a printed feature on an integrated circuit.

FIG. 2B illustrates dissection points and evaluation points on phaseshifter and trim feature edges from FIG. 2A.

FIG. 2C illustrates optical proximity correction on phase shifters andtrim features from FIG. 2B.

FIG. 2D illustrates a printed feature created from the corrected phaseshifters and trim features of FIG. 2C.

FIG. 3A illustrates features associated with corrections in accordancewith an embodiment of the invention.

FIG. 3B illustrates edge dissection and evaluation point placement onthe phase shifters and trim features of FIG. 3A in accordance with anembodiment of the invention.

FIG. 3C illustrates optical proximity corrections on the phase shiftersand trim features of FIG. 3B in accordance with an embodiment of theinvention.

FIG. 3D illustrates a printed feature printed using the phase shiftersand trim features of FIG. 3C in accordance with an embodiment of theinvention.

FIG. 4A illustrates a phase-shift mask in accordance with an embodimentof the invention.

FIG. 4B illustrates a binary mask in accordance with an embodiment ofthe invention.

FIG. 4C illustrates a desired printed feature in accordance with anembodiment of the invention.

FIG. 4D illustrates an actual printed feature in accordance with anembodiment of the invention.

FIG. 5 illustrates shape specification using edge segments and theirjoints in accordance with an embodiment of the invention.

FIG. 6 presents a flowchart illustrating the process of performingoptical proximity correction in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

Overview

In conventional binary photomasks, the geometric mask pattern is usuallydirectly reflected in the intended pattern on the wafer. Therefore, OPCprocesses based on mask images can give adequate compensation to theproximity effect to achieve a desirable wafer image. As we are dealingwith more advanced photomasks, particularly with multiple exposures, theoriginal mask image may not correctly reflect the intended wafer image.In these cases, the conventional OPC strategy, which focuses on maskgeometries, will not result in a good wafer image. Therefore, it isnecessary to focus on the intended wafer images, which could besignificantly different from the mask images. On the other hand, eachmask is exposed individually and will still exhibit mask properties(e.g. corner rounding) therefore it is necessary to compensate forsingle mask effects and obey mask rules.

Mask Features

FIG. 3A illustrates aspects associated with corrections in accordancewith an embodiment of the invention. Areas 302 are line edges of thefeature defined by the desired printed image 120—or alternatively in anoriginal layout prior to its separation into multiple layouts formultiple mask lithographic reproduction. Outlines are shown for phaseshifters 104 and 106 (longer dashed lines) and for trim feature 110(short dashed lines) for reference.

FIG. 3B illustrates dissection and evaluation point placement based onedge features in accordance with an embodiment of the invention. Duringthe optical proximity correction (OPC) process, phase shifters 104 and106, and trim feature 110 are dissected as shown at 304. The dissectionat the shifter outer corners and trim inner corners is done ondissection rules for edges to achieve the best line width and uniformitycontrol, while taking into account mask rule constraints such as minimumserif size. After dissection, evaluation points 306 are placed accordingto evaluation point placement rules for line edges, not corners.

FIG. 3C illustrates the mask layouts after optical proximity correctionsare applied in accordance with an embodiment of the invention. Afterevaluation points 306 have been placed, OPC is performed on phaseshifters 104 and 106 and on trim feature 110. Areas 308 and 310 receivecorrection features for corners, based on tolerances and resistthresholds for line edges. For example, corner serifs are used insteadof edge biases. This gives better corner rounding compensation and henceresults in straighter lines. These corrections obey the mask minimumspacing and width rules for corners.

FIG. 3D illustrates a printed feature 312 in accordance with anembodiment of the invention. Note that the edges at 314 are straighterthan that of FIG. 2D, which shows the result of conventional mask imagebased OPC. Outlines are shown for phase shifters 104 and 106 and fortrim feature 110 for reference.

FIG. 4A and FIG. 4B illustrate a set of double exposure phase shiftmasks to produce the desired printed image shown in FIG. 4C inaccordance with an embodiment of the invention. In FIG. 4A, phase-shiftmask 402 includes 0-degree phase shifter 404 and 180-degree phaseshifter 406. In FIG. 4B, binary mask 408 includes trim feature 410. InFIG. 4C the phase shifters are shown as dashed lines and the trimfeatures are shown as dotted lines for reference. During the firstexposure by the phase-shift mask 402, the dashed edges 414 of theprinted feature will be formed by exposing the resist within the phaseshifter areas 404 and 406. During the second exposure by the trim mask408, the dotted edges 416 and 418 of the printed feature will be formedby exposing the resist outside of the trim area 410.

As shown in FIG. 4C, the desired printed image 412 is a T-shapedstructure, with line end 418. Note that neither the phase shifters 404and 406, nor trim feature 410 alone fully describe desired printedfeature 412. The correct shape of desired feature 412 can be identifiedonly by determining the results of the multiple exposures—or fromlooking at the original layout that was used to create the masks 402 and408. Therefore, the correction parameters that are applied to phaseshifters 404 and 406 and to trim feature 410 can be determined only fromdesired feature 412.

FIG. 4D illustrates an actual printed image 420 by the two masks, priorto applying OPC. In addition to the usual problems caused by proximityeffect such as line printing narrower than desired, there are twoadditional outstanding problem: the line end shortening at 422, and thenecking effect at 428. The line end shortening or pullback is awell-known problem that occurs for all line end features. Line endfeatures are identified by a short line end roof 426, sandwiched by twolonger line end walls 424, where the line end roof is usually no longerthan the maximum line end roof width, and the line end walls are usuallyno shorter than the minimum line end wall height. The photo-resistreacts to this particular kind of image formation to cause theadditional line end shortening. The line end walls are formed by thephase shift mask. However, in the phase shift mask, the line end wallsappear as part of the phase shift edges, and there is no hint of thembeen line end walls of a particular line end. Similarly line end roof isformed by part of the trim edge, but there is no indication of a lineend roof in the trim mask either. In fact, the line end roof appears aspart of a long edge in the trim mask, suggesting it is a line edge, nota line end. Only by looking at the target printed image is this line endfeature obvious. This means that the edge portion of the trim mask 418must be treated as line end, instead of as edge as it may appear in thetrim mask. Consequently, dissection and evaluation point placement,resist threshold, and error tolerance for line end, not for edge, mustbe applied for the edge portion 418.

Similarly, neither the phase shift mask nor the trim mask suggests thenecking effect at location 428. The necking is caused by the T-shapedtarget image from the combination of the two masks and the exposuresystem. One embodiment of the present invention described below inconjunction with FIG. 5 provides a mechanism to identify such shapesfrom the masks and subsequently provides OPC corrections to the maskcorners and edges to reduce this necking effect.

FIG. 5 illustrates shape specification using edge segments and theirjoints in accordance with an embodiment of the invention. The numbers onFIG. 5 correspond to the various edge segments with correspondence toindividual masks while the letters correspond to different dimensions,which characterize the shapes. Edges 520 are the trim mask edges, whileedges 522 are the phase shifter edges. See U.S. Pat. No. 6,523,162 B1,assigned to the assignee of this application for a description of anapproach to shape-based specifications. Once a shape has been identifiedwhich is marked to indicate potential necking problems, the OPC processcan apply special OPC treatments to correct the necking. For example,the phase shift edges and binary edges can be more densely dissected tocapture line width variation and enable more effective correction. Asimilar shape-based approach can be used in the OPC process to handledifficult cases.

FIG. 6 presents a flowchart illustrating the process of performingoptical proximity correction in accordance with an embodiment of theinvention. The system starts by receiving a layout for an integratedcircuit (step 602). Next, the system calculates the target printed imagefrom the layout and the exposure conditions (step 604). This calculationinvolves simple Boolean operations and sizing operations. Shapes arethen defined according to the desired printed image (step 606). A shapeis multiple connected edges with length and angle constraints. Formultiple exposure cases, the shapes on the mask image are typically notthe same shapes on the wafer image.

The system next determines the edges that define the printed image inthe layout (step 608). This is accomplished by matching the target imageand the mask image, determining which edges will be resolved by whichexposure, and then which edges will determine the printed image. Thesystem then associates shapes from the printed image with edges for OPCfrom the layout (step 610). This association is accomplished by matchingshapes in the target image with edges in the masks, which define theprinting. Note that a target shape can be formed from edges fromdifferent masks and not from a single mask. In simple, single exposuremasks, shapes are formed from edges form the same mask.

Next, the system associates target specifications with layout edgesaccording to the printed image-based shapes (step 612). This includesedge dissection (coarse dissection for edges, fine dissection forcorners), evaluation point placement (at center for edges, biased awayfrom vertex for corners), tolerance (tighter on edges, looser oncorners), and threshold (depending on resist properties).

The system then modifies or constrains the dissection and the max/mincorrection based on design rules or mask rules (Step 614). Thismodification includes potential serif size and the max/min correctionamount for edges and corners. The edge or corner will be determined bythe mask, not the target image as before. For example, a shifter outercorner may define an edge on the target image, but OPC will follow thecorner design rule or mask rule.

Finally, the system executes an OPC based on mask features and subjectto design or mask rules (step 616). This includes the correction type(e.g., bias or serif), minimum spacing, and minimum width(corner-to-corner vs. edge-to-edge).

CONCLUSION

The foregoing description is presented to enable one to make and use theinvention, and is provided in the context of a particular applicationand its requirements. It is not intended to be exhaustive or to limitthe invention to the forms disclosed. Various modifications to thedisclosed embodiments will be readily apparent, and the generalprinciples defined herein may be applied to other embodiments andapplications without departing from the spirit and scope of theinvention. Thus, the invention is not intended to be limited to theembodiments shown, but is to be accorded the widest scope consistentwith the principles and features disclosed herein. Accordingly, manymodifications and variations will be apparent. The scope of theinvention is defined by the appended claims.

The data structures and code described in this detailed description canbe stored on a computer readable storage medium, which may be any deviceor medium that can store code and/or data for use by a computer system.This includes, but is not limited to, magnetic and optical storagedevices such as disk drives, magnetic tape, CDs (compact discs) and DVDs(digital versatile discs or digital video discs), and computerinstruction signals embodied in a transmission medium (with or without acarrier wave upon which the signals are modulated). For example, thetransmission medium may include a communications network, such as theInternet. In some embodiments, the electromagnetic wave form includesone or more of the Proteus™ and iN-Tandem™ software programs, both fromSynopsys, Inc., Mountain View, Calif., adapted to perform OPC on phaseshift masks according to the process of FIG. 7.

Note that the invention can be applied to any type of lithographicprocess for fabricating semiconductor chips, including processes thatmake use of, deep-ultraviolet (DUV) radiation, extreme ultraviolet (EUV)radiation, X-rays, and electron beams, along with suitably modifiedmasks.

1. A method that performs target-image-based optical proximitycorrection on masks that are used to generate an integrated circuit,comprising: receiving a plurality of masks that are used to exposefeatures on the integrated circuit; computing a target image for atarget feature defined by the plurality of masks, wherein mask featuresfrom different masks define the target image; dissecting the maskfeature into a plurality of segments, wherein dissecting the maskfeature involves using dissection parameters associated with geometriccharacteristics of the target image, instead of using dissectionparameters associated with geometric characteristics of the maskfeature; and performing an optical proximity correction (OPC) operationon the plurality of masks, wherein the OPC operation uses parametersassociated with geometric characteristics of the target image to performoptical proximity correction on mask features that define the targetimage.
 2. The method of claim 1, wherein different OPC parameters areassociated with line edges, space edges, outer corners, inner corners,line ends, and slot ends of the target image.
 3. The method of claim 1,wherein performing OPC on a mask feature involves placing evaluationpoints on the plurality of segments, and using dissection and associatedevaluation points in performing the OPC operation on the mask feature.4. The method of claim 1, wherein placing evaluation points on the maskfeature involves using evaluation point placement rules associated withgeometric characteristics of the target image, instead of usingevaluation point placement rules associated with geometriccharacteristics of the mask feature.
 5. The method of claim 1, whereinperforming an OPC operation on a mask feature from the plurality ofmasks involves using a resist threshold associated with geometriccharacteristics of the target feature instead of geometriccharacteristics of the mask feature.
 6. The method of claim 1, whereinperforming an OPC operation on the mask feature involves usingtolerances associated with geometric characteristics of the targetfeature instead of tolerances associated with geometric characteristicsof the mask feature.
 7. The method of claim 1, wherein the plurality ofmasks includes at least one phase-shift mask and at least one trim mask.8. The method of claim 1, wherein identifying target image shapeinvolves computing the target image for the shape specification definedby a printed image and which refers back to corresponding mask featuresfrom a plurality of masks.
 9. A computer-readable storage medium storinginstructions that when executed by a computer cause the computer toperform a method that performs target-image-based optical proximitycorrection on masks that are used to generate an integrated circuit, themethod comprising: receiving a plurality of masks that are used toexpose features on the integrated circuit; computing a target image fora target feature defined by the plurality of masks, wherein maskfeatures from different masks define the target image; dissecting themask feature into a plurality of segments, wherein dissecting the maskfeature involves using dissection parameters associated with geometriccharacteristics of the target image, instead of using dissectionparameters associated with geometric characteristics of the maskfeature; and performing an optical proximity correction (OPC) operationon the plurality of masks, wherein the OPC operation uses parametersassociated with geometric characteristics of the target image to performoptical proximity correction on mask features that define the targetimage.
 10. The computer-readable storage medium of claim 9, whereindifferent OPC parameters are associated with line edges, space edges,outer corners, inner corners, line ends, and slot ends of the targetimage.
 11. The computer-readable storage medium of claim 9, whereinperforming OPC on a mask feature involves placing evaluation points onthe plurality of segments, and using dissection and associatedevaluation points in performing the OPC operation on the mask feature.12. The computer-readable storage medium of claim 9, wherein placingevaluation points on the mask feature involves using evaluation pointplacement rules associated with geometric characteristics of the targetimage, instead of using evaluation point placement rules associated withgeometric characteristics of the mask feature.
 13. The computer-readablestorage medium of claim 9, wherein performing an OPC operation on a maskfeature from the plurality of masks involves using a resist thresholdassociated with geometric characteristics of the target feature insteadof geometric characteristics of the mask feature.
 14. Thecomputer-readable storage medium of claim 9, wherein performing an OPCoperation on the mask feature involves using tolerances associated withgeometric characteristics of the target feature instead of tolerancesassociated with geometric characteristics of the mask feature.
 15. Thecomputer-readable storage medium of claim 9, wherein the plurality ofmasks includes at least one phase-shift mask and at least one trim mask.16. The computer-readable storage medium of claim 9, wherein identifyingtarget image shape involves computing the target image for the shapespecification defined by a printed image and which refers back tocorresponding mask features from a plurality of masks.